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Single crystals of PbTiO3 (PT) and BiFeO3(BFO) were grown by a self-flux method, and the influence of lattice defects on the leakage current properties at 25degC was investigated. While PT crystals annealed in air at 700degC showed a leakage current density of the order of 10-5 A/cm2 , annealing under a high oxygen partial pressure of 35 MPa at 700degC increased the leakage current density to 10-4 A/cm2 . This increase in leakage current by the oxidation treatment provides direct evidence that electron hole plays a dominant carrier for the leakage current property in the PT system. The conductivity at 800degC of the PT crystals proportionally increased with an increase in oxygen partial pressure, and electron hole is revealed to be a detrimental carrier even at 800degC. Thermogravimetric analysis showed that a larger weight loss due to PbO vaporization was observed under a higher oxygen partial pressure at high temperatures above 1000degC. The enhanced vacancy formation of Pb under a higher oxygen partial pressure demonstrates that the surface reaction between Pb atoms and O atoms adsorbed onto the PT surface, which leads to PbO (g), is the limiting factor for the vacancy formation in the PT system. It is suggested that Pb vacancies act as an electron acceptor for generating electron holes, leading to a higher leakage current. It is found that the mechanism of the leakage curren for BFO cyrstals is almost the same as that for PT crystals.